This invention relates to a portable add-on fender unit for tractors and/or trailers to provide temporary protection from water spray and debris discharged by the road wheels. The invention is especially suited for use on vehicles where permanent fenders are not feasible due to design constraints on mounting space, vehicle weight, wheel turning action, or interference by other vehicle structure during vehicle turn actions. Principal objects of the invention are to provide temporary add-on fenders that can be manufactured at low cost and applied to a range of tractor-trailer systems without special tools or modifications to the vehicle.